Freediving buoyancy and its effect on energy expenditure

Freediving buoyancy and its effect on energy expenditure

How much energy do we really expend during a dive? This was a question posed by Connor, after our last post on muscle metabolism (Part 1, Part 2). This article is the result of that question. Perfect freediving buoyancy is nearly a science on its own.

How much energy does a dive cost if you are wearing a wetsuit? How much less energy if you decide to dive on an exhale instead of an inhale? In this post, we will look at freediving buoyancy and how much energy you burn going up and down. The results may surprise you!

Try to imagine you are a perfectly average male (sorry ladies). You are the embodiment of averageness. You are 80 kg, (176 lbs), 1m 75 (5’10”) and have approximately 15% body fat and perfectly standard lungs with 1.2 liters functional capacity, 2.3 liters functional residual volume, 5.8 liters normal full lungs and 8.5 liters packed lung capacity. I realize that there is a small chance that you are not, for whatever unfortunate reason, completely average. That’s ok, even though body composition matters, lung inflation and the thickness of your wetsuit matters even more.

Let’s have a look at what affects freediving buoyancy.

Freediving buoyancy: body composition

Your body composition has a significant effect on your buoyancy. Fat is somewhat buoyant, muscle is somewhat negatively buoyant and bones are very negatively buoyant. Other soft tissues have a density close to that of water. The density of the body without any gas in the lungs (or intestines) is somewhere in between 1.01 – 1.08 g / cm3. This is always heavier than fresh water, and usually heavier than salt water. Our average diver has a density of 1.05 g / cm3.

Freediving buoyancy: the lungs

The lungs are compressible, and this causes their effect on buoyancy to change with depth. There will always be a net positive buoyance from the lungs. This positive buoyancy is great at the surface, but small at depth.

Freediving buoyancy
The body has a specific buoyancy (x-axis) which does not change with depth (y-axis). You can see that the net effect of the lungs is always a positive buoyancy force. However, the magnitude of this force declines drastically with increasing depth. The change in buoyancy is largest close to the surface.

Freediving buoyancy: neoprene

Your wetsuit is made of neoprene. Neoprene without any bubbles in it has a density of approximately 1.3 g/cm3. The wetsuit is made supple and insulating by injecting the neoprene with nitrogen. All these nitrogen bubbles add buoyancy. Neoprene can contain anywhere from 30% to 94% nitrogen. More nitrogen means a stretchier wetsuit that is more insulating at the surface, but less insulating at depth. Here I assume a neoprene nitrogen content of 84%, and a base density of 1.3 g/cm3, leading to a final density of 0.24 g/cm3. These numbers were obtained with some help from friends at Azure Passion. (I am trying to get more detailed specs on a variety of neoprenes, if I succeed, it will be added or linked to here).

Weight, or uncompressible buoyancy?

The last variable, which is the easiest for us to change, is how much weight we carry with us. Some divers also decide to take down uncompressible buoyancy. This basically counts as negative weight. I dive with 600 grams of positive buoyancy (incompressible plastic spheres) in my fin, in addition to 13 pounds of lead on my neck and waist, so that my legs float and I can stay vertical during my descent. But how much weight should we use in total?

freediving buoyancy
Neoprene has an effect on buoyancy that is similar to that of the lungs. Neoprene is always positively buoyant (in a freedivers depth range). Lead of course has a resulting negative buoyancy force, which is constant.

Energy cost of freedives

We should be weighing ourselves so that we can minimize the work we do during a dive. Work is defined in physics as ‘W = F x a, or work equals a force times a distance. If the buoyancy force of your body, Fb = 20 during your entire dive, and you dive to 20 m, you have to expend 20 x 20 = 400 joules (approximately 50 calories) to dive down to 20 meters.

On the graphs in this article, the distance (depth) is plotted on the y-axis, and the force resulting through buoyancy is plotted on the x-axis. The area underneath the curve is the amount of work that a diver has to do to get to a specific depth. This makes intuitive sense, because the higher or lower a buoyancy force is the more work has to be done in order to counter that force. If a diver is very negatively buoyant, it costs a lot of effort to come to the surface. If a diver is very positively buoyant, it will cost a lot of effort to dive down.

In the following graphs you can see a hypothetical 130 m dive, which is close to the current CWT record of 129 m. These curves are for divers with full lungs, and a variable suit. The divers have been perfectly weighted in order to minimize the area under the curve.

freediving buoyancy
Here are a set of hypothetical 130 m dives by our average diver. The suit is variable. If the line is close to x = 0 (indicated with the top arrow), the buoyancy is small and the total energy expenditure is small too. The simple conclusion is that a dive with no suit is much more energy efficient than a dive with an 8 mm suit. A more interesting conclusion is that in order to be weighted properly, divers need to bring positive buoyancy with them, rather than lead weight (which is negatively buoyant). The amount of weight required is indicated on the legend. The minus sign indicates that the weight is negative!

An interesting thing to note is that all divers are neutrally buoyant at half their target depth. This is a recurring finding, for any depth, and holds for any combination of suits, lung fill, body types, and weight.

Another interesting thing to note is that without using incompressible buoyancy,  all divers (except the diver with the 8 mm suit) are too negatively buoyant in order to be diving to 130 m. They will reach neutral buoyancy too early, and hence waste effort on the ascent. For example, our no suit diver requires 1.2 kg of positive buoyancy in order to be perfectly weighted.

A no suit diver expends approximately 822 J (200 cal) on the ascent and descent if perfectly weighted (1.2 kg positive buoyancy). If the diver decides to carry no weight, this increases to 1500 J.

Of  course no freediver in his/her right mind would attempt a 130 m dive in an 8 mm suit. The buoyancy force of a ‘perfectly weighted diver’ in an 8 mm suit at the surface is so high, that it would be close to impossible to dive down to any depth. The diver has to strap on half a ton of weight to make the descent possible. All this weight makes the ascent next to impossible.

Energy cost of 25 m freedives

I know, you don’t dive to 130 m, and I don’t dive to 130 m either. So what do people other than Alexey Molchanov and Guillaume Néry take away from this study? My last recreational dive session averaged 24.5 m, so I will focus on 25 m dives here.

Here is a plot for divers that dive on full lungs with a variety of suits:

freediving buoyancy
The buoyancy force plotted against depth for 25 m dives with full lungs and a variable suit. Note that diving without a suit is much more energy efficient than diving with an 8 mm wetsuit.

Keep in mind that the area in between the curve and the y-axis corresponds to the total energy expended. Because the curve is far away from the axis above neutral buoyancy (12.5 m), this is where we lose most of our energy.

And here is plot for divers with a 3 mm suit, that dive on 1) a forced exhale, 2) a passive exhale, 3) full lungs, and 4) packed lungs:

freediving buoyancy
The buoyancy force plotted against depth for 25m dives with variable lung inflation and a 3 mm wetsuit. Note that diving with packed lungs costs nearly as much energy as a dive with full lungs with an 8 mm wetsuit! FV  implies a forced exhale.

What do we learn from this? Well, the first lesson is one that you probably already know. Your suit should be as thin as possible, because it takes a lot of effort to dive with a thick suit. Second lesson, if you bring down less air your dive will be more energy efficient. Yes, you read that correctly. A forced exhale is much more energy efficient than packed lungs, and the difference is big. Our diver expends less than half of the energy diving after a forced exhale, as opposed to diving after packing. Apart from that, after an exhale, our diver only needs 0.3 Kg to be perfectly weighted, and after packing our diver needs  3.6 Kg!

If you do dive with a thick suit you can offset the extra buoyancy by diving FRC (functional residual capacity, this is diving after a passive exhale). Note that a diver with packed lungs and a 3 mm suit nearly expends as much energy as a diver with an 8 mm suit with full lungs! A diver with an 8 mm suit on FRC expends approximately 476 J on overcoming buoyancy, and a diver with a 3 mm suit and packed lungs expends 465 J on a 25 m dive.

Of course, the significant downside to diving FRC or even RV (residual volume, or diving after a forced exhale) is that you will not carry as much oxygen with you. Some divers feel that the sacrifice is worth it. However, I do not know of any recent records that have been set with an FRC dive, so it appears that oxygen (or perhaps equalization) is a limiting factor for dives to great depth.

Physiology

So far you’ve learned that you will be most efficient, theoretically, if you weigh yourself at half of your target depth. If you want to dive deep, especially with your thick  suit, your physiology likely won’t allow that. First of all, it will take a monstrous amount of energy to get down to neutral buoyancy because the buoyancy force at the surface is higher than the negative buoyancy force at the surface. In colloquial terms, you float better than you sink. This is the expected consequence of the exponential decrease in volume of the gas in your lungs and neoprene. It is also the main reason you need a decent amount of weight at the start of the dive.

Looking closely at the graphs, there is another important reason for using weight. The negative buoyancy force might be so low that you will not sink quickly enough. If you look at the ‘no suit, 130 m’ graph, you will see that our diver needs to wear 1.2 kg of incompressible bouyancy. As a result, the resulting negative buoyancy at depth is so weak that the diver will likely sink too slowly.

Unfortunately, our physiology and the laws of physics do not always align. See our article on adaptation here.

Practical tips

  • Aim for neutral buoyancy at half of your target depth. During a dive session in which you dive 20 – 30 m, with 25 m dives on average, aim to be neutral at 12.5 m. A more conservative approach is to be neutral at half of your maximum expected depth (15 m). If you pack, and/or use a thick suit, this is especially important.
  • If you have no problems with lung or trachea squeezes, explore FRC or even RV diving. You will produce less CO2 and will need less weight. Several threads on the Deeper Blue forums are devoted to FRC diving. RV diving has recently been suggested by Aharon Solomons as a method to acclimatize to depth.

Always dive with a buddy and be very careful. If you dive on less than full lungs you are missing out on a large oxygen reservoir and are more prone to squeezes.

 

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Jaap

Jaap is a geologist by trade and a freediver by passion. Jaap wrote the book Longer and Deeper in 2018. His book teaches how to train for freediving and spearfishing on land.

This Post Has 22 Comments

  1. Luca M.

    Good article! I suggest to use the term RV (residual volume) rather than FV (forced exhale)…the acronym FV sounds like full volume.

    Also, in this paragraph;

    ”Another interesting thing to note is that all divers (except the 8mm diver) are too negatively buoyant in order to be diving to 130 m. They will reach neutral buoyancy too early, and hence waste effort on the ascent. For example, our no suit diver requires 1.2 kg of positive buoyancy in order to be perfectly weighted.”

    did you then mean to say that the diver should have been neutrally buoyant at c. 65-70m? So they find the soft spot between being not too buoyant in the descent, but then not too heavy in the ascent?

    1. Jaap

      Hi Luca, you are totally right. I believe I have changed it all to RV now. On all dives, regardless of the thickness of the suit or the buoyancy of the body etc., the total energy cost is minimized if the diver is neutral at mid depth. This means that a 130 m diver will want to weight themselves so they are neutral at 65 m. For our average diver (and most other human beings) this implies diving with positive buoyancy, rather than with lead. An early (comfortable) sink phase comes at a cost.

      In all the graphs, the total energy cost has been minimized, and that means that a no suit diver requires 1.2 kg of positive buoyancy (to be precise, 12 newton-meters of positive buoyancy from incompressible plastic).

      This was one of the reasons that Eric Fattah added positive buoyancy to the Orca DOL-fin monofin. It would minimize the energy cost of deep dives.

      I realize I was perhaps somewhat unclear in that part of the article, so I’ve edited it accordingly.

  2. Connor

    OUTSTANDING!! What a great analysis. Thank you!! Freediving needs more of this kind of stuff.

    These kinds of numbers help tremendously in developing freediving technique. Among lots of other things, it illustrates one of the main reasons diving FRC is so effective. I knew that, but your numbers provide major comfirmation.

    The idea that neutral bouyancy should be at half the depth is a new one to me. I dive half lung and always set my neutral depth at about 5 meters, so as to minimize effort on descent. That works great when diving 15-20 m, but I (and some of my dive buddies) had noticed that coming up from 25+m was often hard. Plan to experiment with deeper neutral in the future.

    There is an additonal factor not considered above. Keeping effort low during the first half of the dive gives DR a chance to set in strong and changes the 02 burn pattern, conserving 02. That effectively means, for these dives where the diver isn’t fighting to get down, the most effcient place for neutral bouyancy will be less than half way down. Looks to me like for deeper dives I still need to set a deeper neutral point. This is a tricky one that will take some experimentation.

    I was surprized at the size of the joule numbers. Getting the weighting right and minimizing excess bouyancy is way more critical than I thought.

    Here is an idea to calculate. An half lung diver, diving 20 meters, neutral at 5 meters, who expends no more energy than a static when descending, other than a surface dive, arm pull and two soft kicks. How does that compare to a full lung no suit diver neutral at 10 meters.

    I added bouyancy to my dolfin as well, balance with a bit more lead in the neck weight.

  3. Connor

    In at least two of the graphs, it appears that the 5 mil suit requires more effort than the 8 mil. Am I interpreting this correctly? If so, why is that?

    1. Jaap

      My mistake. I mixed up the legend of the graphs. The 8 mm would require more effort, thanks for picking up on that. I have changed that.

      If you are able to dive down only doing a duck dive (neutral at 0 m, negative after a duck dive) and dive FRC with a 3 mm suit, you expend about 414 J or ~100 cal more on a 25 m dive than a diver that is weighted to be neutral at 12.5 m. Your total energy expenditure would be (drum roll) about 600 J, which is nearly 3 times higher than for a diver that is neutral at mid depth. I was surprised by these numbers too.

      The problem is of course that if you want to do one dive to 30 m, but the majority in the 10 – 15 m range, you can’t be neutral at 15. In addition to that our biology (onset of the DR and the early production of CO2 during a more difficult descent) may not allow us to weigh ourselves for an ideal situation.

  4. Jaap

    I will write a post in the future on the effects of over-weighting.

  5. Gabe

    Good read, thanks.

    Instead of weighting to be neutral at half your depth, why not weight to use the least amount of energy? It looks like it would be good to be neutral at a shallower depth. The area under the buoyancy force curve (“energy expended”) should be reduced by shifting your weighting to be neutral at a shallower depth.

    Our am I miss understanding something here? The lack of oxygen makes it possible!

    1. Jaap

      Hi Gabe, the energy expended is already minimized in all the graphs. I did this manually, by tweaking the amount of weight carried. For all situations the energy expenditure is minimized if the diver is weighted at half depth! I was surprised by this result too, but the numbers don’t lie. Of course there may be practical reasons that do not allow us to weigh ourselves for neutral buoyancy at mid-depth.

  6. Lance Lee Davis

    This is a very interesting analysis, thanks for taking the time to do it. I do think one major thing is missing though in regards to real world application–and this would vary among different divers–is that rate of blood O2 burn during active descent (pre-freefall) can be very, very high. For myself at least, my muscles are working largely aerobically at this stage and pulse is high. So weighting extremely light may reduce overall energy expenditure, but it will cost me more blood O2 over the course of the dive, which in turn can bring me closer to blackout…

    1. Jaap

      Hi Lance, you are totally right, most of us agree that the O2 cost of effort at the start is higher than at the end of a dive. Perhaps the question is, how do we kickstart the dive reflex *before* we start descending? See also the discussion here: https://freedivewire.com/muscle-metabolism-freediving/

  7. Connor

    Hmmm, tricky to model my kind of dive, lots of variables interacting. A more precise model should show that what I’m doing is not as energy inefficient as the example. That said, the results in the article are compelling. I’m pretty sure that I and a lot of other divers need to be adjusting our neutral point to a bit deeper. Just how much is hard to say.

    I intend to experiment with this concept, adjusting neutral depth with dive time as my dependent variable. I’m fairly consistent on dive time and any significant change in efficiency should show up in time down. Any thoughts on this?

    1. Jaap

      I think it’s exactly as you say, just experiment. Perhaps weigh yourself for neutral buoyancy at 10 m for the next 3 dive sessions, check the results, then 12.5 m, 15 m, and once the dive times reduce, take on some more weight. It will probably feel quite a bit different. Let us know the results! One thing we simply don’t know right now, is how many Joule we expend on a dive. Perhaps the amount of energy we lose is completely negligible compared to the energy lost through heat transfer… In which case your dive times will remain the same. If the DR trumps the buoyancy requirements, your dive times will reduce.

  8. Steven

    There is an issue in the following passage:

    “It may not seem like much but our average Joe would have to walk for about an hour to burn this much energy.”

    The author has mixed up calories and kCal (kilocalories). The confusion might be due to the common use of the word “Calories” to describe kCal in North America.

    The described energy expenditure (822 Joules / 200 calories / 0.2 kCal) is about same as walking for a few seconds.

    1. Jaap

      Thanks for noting that Steven, edited accordingly. That’s why I try to stick to SI units… I still get lost in translation.

  9. k

    Variable weight could benefit from positive buoyancy. Like wearing 8mm suit, but then the movement would be restricted as hell. How about using regular suit and stuff pieces of wood around the arms, legs, butt to set WR in variable weight? I guess it shouldn’t be allowed right? Different disciplines have different optimal weighting strategy, for example, quick deep free immersion men can prefer neutral less than 10m for 100m dive, but they would love to be neutral at 20m in monofin, because pulling rope doesn’t seem to be a big deal when it’s a little heavier.

    1. Jaap

      Hi K,

      I think it is allowed in VWT. You may still need to use your arms and the ideal situation will take some experimenting, but in theory you will require less energy. Of course streamlining might also become and issue. Interesting thoughts about the different disciplines, I like it!

  10. Halfdan Hem

    This is really interesting Jaap, thanks for a great article and thorough breakdown and analysis.

  11. WIldor Di Novo

    Very interesting topic and article!
    I guess it would be nice to find a way to factor in dive time vs O2 consumtion into this analysis as well.
    If the dive time becomes significantly longer for a diver by following the above analysis then perhaps a faster descent with more weight might be worth pursuing. The increasingly anaerobic state the body goes into during a dive (plus dive response) also allow for less O2 consumption during the ascent even if you have to use more energy.
    I’m aware this is all slightly beyond the point of the article which looks at buoyancy/force relation alone. But it nevertheless seems interesting enough to me to bring it up and perhaps one day see a full analysis of that sort 🙂
    Cheers and thank you again!

    1. Jaap

      Hi Wildor, thanks, glad you enjoyed the article! You are totally on point, this analysis only looks at buoyancy and force. However, our physiology does not always allow for the buoyancy/force to be optimized. I dive with an 8 mm suit in cold water with a lot of weight, and if I try to be neutral any lower than 12 m, there is no chance in the world that I will get down in the first place! Weighing myself optimally for a 40 m dive (neutral at 20 m) is quite impossible. You’re probably right too that the negative force on a diver with a thin suit and little weight might be too low to overcome drag. The diver would take ages to get down! If I ever get my hands on enough data to assess this in combination with the dive response (we can dream ;)) I definitely will 🙂

  12. Tsubasa

    very interesting article!
    let me just indicate a few trivial mistakes.

    W = F x l(length) or d(distance)
    “a” indicates acceralation in physics.

    unit of buoyance force in the figures should be “N”.
    it should be capital as it is namef after Newton and “n” is usef for an unit prefix of nano.

    anyway, it is a great article!

  13. Filip Schagatay

    Hi,

    I have to say that i like the effort behind this article.

    I do however have some quite large points to make in relationship to this article. And frankly i think it should be removed since it is more confusing than informative.

    1 Your statement about the area under the curve is only valid if the individual is moving at the same speed during the whole dive. But if you would look at the time an actual athlete would spend at each depth under the different buoyancy conditions you would see that the time spent fighting the higher buoyancy at the surface would be significantly grater than the time spent at less buoyant depths. You would need to have a curve over time spent at each depth during a dive with each buoyancy to be able to say anything about how much energy is expended by an athlete.

    2 You are disregarding the physiology of free diving. Having neurtra buoyancy at around 10 meters depth allows you to totally relax, meditate, lower your cognitive workload while free falling. During this time diving response kicks in with peripheral vasoconstriction and bradycardia. At rest brain activity accounts for 30% of metabolism and meditating this is ment to decrease further. Then at max depth you start working, but vasoconstriction i maintained. Your muscles work almost anaerobically. You consume less oxygen than you would have if you did the same work without the dive response in place. And oxygen is the key factor to come up with consciousness intact. While work during the way is interesting to look at it must be done in a physiological context. Now starting while having to paddle down to half your target depth means fighting all these responses that allows you to preserve oxygen and it will not do anyone a favor.

    I am however open to having misinterpreted some things so feel free to explain them if it seems that i have.

    P.S. The unit of measurement for the force of buoyancy is N and not Nm.

    Best regards
    Filip Schagatay

    1. Jaap

      Hi Filip, thanks for your thoughtful comments.

      1. I have not attempted to make an estimate of the total energy expenditure of freedivers, just of the energy expenditure that is an effect of being at variable buoyancies during the dive. For the purpose of these calculations, speed does not matter. Of course you are correct that in practice our speed matters. (On another note, I have gone through my depth graphs for CWT dives up to 50m in a variety of conditions and my speed is fairly constant throughout those dives. )

      2. You are correct. As I mention in the article, our physiology doesn’t always agree with simple physics. But note that for a diver going to 80m in warm water, being neutral at 20m still allows for a long sink phase. The freefall depends on the diver as does the relative proportion of anaerobic versus aerobic work. A larger degree of anaerobic work is great until muscle fatigue becomes a larger impediment than hypoxia (I’ve had some issues with this myself).

      Is the brain activity decrease during meditation based on academic work or an assumption? I recently did a modest search and couldn’t find a clear answer.

      You’re right on the units – and not the first to point it out – I should really update the graphs.

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